Lateral geniculate nucleus
The lateral geniculate nucleus (LGN) is the primary relay center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain. The LGN receives information directly from the ascending retinal ganglion cells via the optic tract and from the reticular activating system. Neurons of the LGN send their axons through the optic radiation, a direct pathway to the primary visual cortex. In addition, the LGN receives many strong feedback connections from the primary visual cortex. In mammals, including humans, the two strongest pathways linking the eye to the brain are those projecting to the LGNd (dorsal part of the LGN in the thalamus), and to the superior colliculus (SC).Goodale, M. & Milner, D. (2004)Sight unseen.''Oxford University Press, Inc.: New York. Structure Both the left and right hemisphere of the brain have a lateral geniculate nucleus, named so for its resemblance to a bent knee (''genu is Latin for "knee"). In many primates, including humans and macaques, it has layers of cell bodies with layers of neuropil in between, in an arrangement something like a club sandwich or layer cake, with cell bodies of LGN neurons as the "cake" and neuropil as the "icing". In humans and macaques the LGN is normally described as having six distinctive layers. The inner two layers, 1 and 2, are called the magnocellular layers, while the outer four layers, 3, 4, 5, and 6, are called parvocellular layers. An additional set of neurons, known as the koniocellular sublayers, are found ventral to each of the magnocellular and parvocellular layers.Carlson, N. R. (2007)Physiology of Behavior: ninth edition.''Pearson Education, Inc.: Boston. This layering is variable between primate species, and extra leafleting is variable within species. M, P, K cells *Size relates to cell body, dendritic tree and receptive field The magnocellular, parvocellular, and koniocellular layers of the LGN correspond with the similarly named types of ganglion cells. Koniocellular cells are functionally and neurochemically distinct from M and P cells and provide a third channel to the visual cortex. They project their axons between the layers of the lateral geniculate nucleus where M and P cells project. Their role in visual perception is presently unclear; however, the koniocellular system has been linked with the integration of somatosensory system-proprioceptive information with visual perception, and it may also be involved in color perception. The parvo- and magnocellular fibers were previously thought to dominate the Ungerleider–Mishkin ventral stream and dorsal stream, respectively. However, new evidence has accumulated showing that the two streams appear to feed on a more even mixture of different types of nerve fibers.Goodale & Milner, 1993, 1995. The other major retino–cortical visual pathway is the tectopulvinar pathway, routing primarily through the superior colliculus and thalamic pulvinar nucleus onto posterior parietal cortex and visual area MT. Ipsilateral and contralateral layers Both the LGN in the right hemisphere and the LGN in the left hemisphere receive input from each eye. However, each LGN only receives information from one half of the visual field. This occurs due to axons of the ganglion cells from the inner halves of the retina (the nasal sides) decussating (crossing to the other side of the brain) through the optic chiasm (''khiasma means "cross"). The axons of the ganglion cells from the outer half of the retina (the temporal sides) remain on the same side of the brain. Therefore, the right hemisphere receives visual information from the left visual field, and the left hemisphere receives visual information from the right visual field. Within one LGN, the visual information is divided among the various layers as follows:Nicholls J., et al. From Neuron to Brain: Fourth Edition. Sinauer Associates, Inc. 2001. * the eye on the same side (the ipsilateral eye) sends information to layers 2, 3 and 5 * the eye on the opposite side (the contralateral eye) sends information to layers 1, 4 and 6. A simple mnemonic for remembering this is "See I? I see, I see," with "see" representing the C in "contralateral," and "I" representing the I in "ipsilateral." Another way of remembering this is 2+3=5, which is correct, so ipsilateral side, and 1+4 doesn't equal 6, so contralateral. This description applies to the LGN of many primates, but not all. The sequence of layers receiving information from the ipsilateral and contralateral (opposite side of the head) eyes is different in the tarsier.Rosa MG, Pettigrew JD, Cooper HM (1996) Unusual pattern of retinogeniculate projections in the controversial primate Tarsius. Brain Behav Evol 48(3):121–129. Some neuroscientists suggested that "this apparent difference distinguishes tarsiers from all other primates, reinforcing the view that they arose in an early, independent line of primate evolution".Collins CE, Hendrickson A, Kaas JH (2005) Overview of the visual system of Tarsius. Anat Rec A Discov Mol Cell Evol Biol 287(1):1013–1025. In visual perception, the right eye gets information from the right side of the world (the right visual field), as well as the left side of the world (the left visual field). You can confirm this by covering your left eye: the right eye still sees to your left and right, although on the left side your field of view may be partially blocked by your nose. In the LGN, the corresponding information from the right and left eyes is "stacked" so that a toothpick driven through the club sandwich of layers 1 through 6 would hit the same point in visual space six different times. LGN inputs The LGN receives input from the retina. At least in some species, the LGN also receives some inputs from the optic tectum (also known as the superior colliculus).In Chapter 7, section "The Parcellation Hypothesis" of "Principles of Brain Evolution", Georg F. Striedter (Sinauer Associates, Sunderland, MA, USA, 2005) states, "...we now know that the LGN receives at least some inputs from the optic tectum (or superior colliculus) in many amniotes". He cites "Wild, J.M. 1989. Pretectal and tectal projections to the homolog of the dorsal lateral geniculate nucleus in the pigeon—an anterograde and retrograde tracing study with cholera-toxin conjugated to horseradish-peroxidase. Brain Res 489: 130–137" and also "Kaas, J.H., and Huerta, M.F. 1988. The subcortical visual system of primates. In: Steklis H. D., Erwin J., editors. Comparative primate biology, vol 4: neurosciences. New York: Alan Liss, pp. 327–391. LGN output Information leaving the LGN travels out on the optic radiations, which form part of the retrolenticular limb of the internal capsule. The axons that leave the LGN go to V1 visual cortex. Both the magnocellular layers 1–2 and the parvocellular layers 3–6 send their axons to layer 4 in V1. Within layer 4 of V1, layer 4cβ receives parvocellular input, and layer 4cα receives magnocellular input. However, the koniocellular layers (in between layers 1–6) send their axons to layers 4a in V1. Axons from layer 6 of visual cortex send information back to the LGN. Studies involving blindsight have suggested that projections from the LGN not only travel to the primary visual cortex but also to higher cortical areas V2 and V3. Patients with blindsight are phenomenally blind in certain areas of the visual field corresponding to a contralateral lesion in primary visual cortex; however, these patients are able to perform certain motor tasks accurately in their blind field, such as grasping. This suggests that neurons travel from the LGN to both the visual cortex and higher cortex regions. Function in visual perception The functions of the LGN are multiple. Its unique folding contributes to its utility by performing a range of anatomical calculations without requiring mathematical computations. These include both temporal correlations/decorrelations as well as spatial correlations. The resulting outputs include time correlated and spatially correlated signals resulting from summing the signals received from the left and right semifields of view captured by each of the two eyes. These signals are correlated in order to achieve a three-dimensional representation of object space as well as obtain information for controlling the precision (previously auxiliary) optical system (POS) of the visual modality. The outputs serve several functions. * A signal is provided to control the vergence of the two eyes so they converge at the principle plane of interest in object space. * A signal is provided to control the focus of the eyes based on the calculated distance to the principle plane of interest. * Computations are achieved to determine the position of every major element in object space relative to the principle plane. Through subsequent motion of the eyes, a larger stereoscopic mapping of the visual field is achieved.Lindstrom, S. & Wrobel, A. (1990) Intracellular recordings from binocularly activated cells in the cats dorsal lateral geniculate nucleus Acta Neurobiol Exp vol 50, pp 61–70 ** A tag is provided for each major element in the central 1.2 degree field of view of object space. The accumulated tags are attached to the features in the merged visual fields forwarded to area 17 of the cerebral cortex (often described as the "primary" visual cortex or V1) ** A tag is also provided for each major element in the visual field describing the velocity of the major elements based on its change in coordinates with time. ** The velocity tags (particularly those associated with the peripheral field of view) are also used to determine the direction the organism is moving relative to object space. These position and velocity tags are extracted prior to the information reaching area 17. They constitute the major source of information reported in blindsight experiments where an individual reports motion in a portion of the visual field associated with one hemisphere of area 17 that has been damaged by laceration, stroke, etc. The output signals from the LGN determine the spatial dimensions of the stereoscopic and monoscopic portions of the horopter of the visual system.Fulton, J. (2004) Processes in Biological Vision Section 7.4 http://neuronresearch.net/vision/pdf/7Dynamics.pdf/ It has been shown that while the retina accomplishes spatial decorrelation through center surround inhibition, the LGN accomplishes temporal decorrelation.Dawei W. Dong and Joseph J. Atick, Network—Temporal Decorrelation: A Theory of Lagged and Nonlagged Responses in the Lateral Geniculate Nucleus, 1995, pp. 159–178. This spatial–temporal decorrelation makes for much more efficient coding. However, there is almost certainly much more going on. Like other areas of the thalamus, particularly other relay nuclei, the LGN likely helps the visual system focus its attention on the most important information. That is, if you hear a sound slightly to your left, the auditory system likely "tells" the visual system, through the LGN via its surrounding peri-reticular nucleus, to direct visual attention to that part of space. The LGN is also a station that refines certain receptive fields. Experiments using fMRI in humans reported in 2010 that both spatial attention and saccadic eye movements can modulate activity in the LGN. Additional images Image:Constudthal.gif|Thalamus Image:Gray683.png|Dissection of brain-stem. Lateral view. Image:Gray722.png|Scheme showing central connections of the optic nerves and optic tracts. Image:ThalamicNuclei.svg|Thalamic nuclei Image:ERP_-_optic_cabling.jpg|3D schematic representation of optic tracts References External links *Malpeli J. Malpeli Lab Home Page. Retrieved September 1, 2004. * * —"The Visual Pathway from Below" * * See also * Medial geniculate nucleus—MGN processes auditory information Category:Cerebrum Category:Visual system